The present invention relates to the fabrication of integrated circuit ("IC") devices, and more particularly to non-uniform local oxidation to achieve differential field oxide thicknesses.
Local oxidation of silicon ("LOCOS") is one type of method used to laterally isolate one device on an integrated circuit substrate from another device on the integrated circuit substrate. In a conventional LOCOS process, a layer of patterned silicon nitride is used as a mask in a thermal oxidation process. Even a thin layer of silicon nitride will prevent significant oxidation from occurring beneath it. The patterned silicon nitride layer allows field oxide to grow in the "window" regions while inhibiting oxide growth in the regions covered by silicon nitride. Unfortunately, oxide growth may occur underneath the edge regions of the silicon nitride layer to form what is commonly known as a "bird's beak".
While silicon nitride is effective in preventing oxygen from diffusing through it to the underlying silicon substrate, and hence preventing the formation of an oxide layer, oxygen can still diffuse along the interface between the silicon nitride and the substrate. In some instances, a layer of pad oxide underlies the silicon nitride to reduce stress-related defects in the IC, and this pad layer can also act as a conduit for oxygen. In either instance, a bird's beak may form underneath the silicon nitride layer.
A desirable characteristic of the LOCOS process is that, after the field oxide is formed, the patterned silicon nitride layer can be stripped from the substrate to form what will become active cells between the regions of field oxide. These active cells are self-aligned to the field oxide, thus making efficient use of the valuable substrate area. Unfortunately, a bird's beak intrudes into the active cell region, reducing the area available for active device fabrication. As device geometries continue to shrink, the relative portion of the active cell area consumed by a bird's beak increases, decreasing the ultimate device density of the IC.
Multi-voltage ICs have an additional problem relating to the formation of bird's beaks. Many devices, such as dynamic random-access memories ("DRAMs") and flash electronically erasable, programmable read-only memories ("flash EEPROMs") use more than one voltage during operation. A low voltage may be used for one type of operation, such as a read/write or sense operation, while a higher voltage is used for a word-line boost operation or a floating-gate program/erase operation. ICs with integrated functions, such as memory and data processing functions, may also operate a more than one voltage. It is generally desirable to minimize the cell size, also known as the design rule, to decrease the IC size and hence cost for a given circuit. However, the ability of field oxide to isolate a voltage is generally related to the thickness of the field oxide. Unfortunately, the size of a bird's beak is generally related to the thickness of the field oxide that is grown. Therefore, field oxide that is thick enough to withstand the higher voltage results in an undesirably large bird's beak intruding into the active cell area of the lower voltage devices. Conversely, a field oxide optimized for the design rule of low-voltage cells might not reliably isolate high-voltage cells.
Therefore a multi-voltage IC with field oxidation that reliably isolates high voltages while allowing tighter design rules for low-voltage cells is desirable.